1. Field of the Invention
The present invention relates to a developing apparatus used for an image forming apparatus such as a copying machine, a printer or the like, and, more specifically, to a developing apparatus and image forming apparatus in which electric charges generated by gas electrolytic dissociation are used for charging a developer.
2. Description of the Background Art
A structure of a developing apparatus used for a conventional image forming apparatus in accordance with electrophotography will be described with reference to FIG. 10. FIG. 10 shows a schematic structure of a developing apparatus applied to mono component developing method using a mono component developer consisting of toner only, in a conventional electrophotographic image forming apparatus.
Opposing to a photoreceptor drum 1 as an image carrier, a developing apparatus 4 for visualizing a latent electrostatic image formed on a surface of photoreceptor drum 1 is arranged. Developing apparatus 4 generally has a rotatable developing roller 41 provided opposing to a developer tank 40, and especially to an opening thereof, which tank contains a toner 10 which is an insulative developer. Developing roller 41 is arranged such that a portion thereof is exposed through the opening of developer tank 40 to be in contact, for example, with photoreceptor drum 1. This contact area serves as the developing area.
Mono component toner 10 is supplied by a supply roller 42 and absorbed by the surface of developing roller 41. In order to regulate the amount of toner absorbed by developing roller 41, a regulating member 43 is provided, in pressure-contact with the surface of developing roller 41. The toner absorbed by developing roller 41 has its amount regulated to a constant value, as it passes through the pressure-contact portion of regulating member 43. As it passes through the pressure-contact portion of regulating member 43, toner 10 absorbed by developing roller 41 is charged, by the friction with the regulating member 43.
Thereafter, toner 10 is conveyed to the developing area opposing to photoreceptor drum 1, while it is carried on the surface of developing roller 41. Toner 10 is then selectively adhered to photoreceptor drum 1 and developed, corresponding to the latent electrostatic image formed on the surface of photoreceptor drum 1.
After development, toner 10 that has not been used for development is conveyed to developer tank 40. In developer tank 40, a supply roller 42 is provided, in pressure-contact with developing roller 41, in order to remove and recover the toner 10 which was not used for development, from the surface of developing roller 41. Toner 10 that was not used for development and carried on the surface of developing roller 41 is scraped off by supply roller 42. Further, by supply roller 42, toner 10 is newly supplied to the surface of developing roller 41.
In order to ensure satisfactory development, generally, a developing bias voltage is supplied to developing roller 41. The developing bias voltage is set to such a voltage value that ensures adhesion of toner 10 on the latent electrostatic image at the time of development while toner 10 is not adhered to portions other than the latent electrostatic image on photoreceptor drum 1.
In order to apply a prescribed amount of electric charges of a prescribed polarity to the toner 10 absorbed by developing roller 41, a regulating voltage is supplied from regulating member 43. Therefore, as toner 10 passes through the pressure-contact portion of regulating member 43, the amount of the toner is made constant, the toner is friction-charged, and the toner thus charged by a prescribed amount with a prescribed polarity is conveyed to the developing area.
As described above, the mono-component toner as the developer is absorbed by the developing roller and conveyed to the developing area, and the toner is adhered to the latent electrostatic image on the photoreceptor drum, whereby an image is formed.
In the developing apparatus 4 to which the method of friction charging described above is applied, the charge amount of the toner charged by the regulating member 43 does not obtain to a sufficiently saturated state, and therefore, charge amount varies among toner particles. Further, there arc uncharged toner particles resulting from mis-contact with the regulating member 43, as well as toner particles charged to a polarity opposite to the desired polarity (hereinafter referred to as reverse-charged toner) that are inevitable in friction charging.
As a result, the developer comes to have wide distribution of charge amount, degrading stability of development. Particularly, it follows that the reverse-charged toner develops a portion which is inherently a non-image portion, and hence quality of the image is degraded. Further, an average value of charge amount significantly differs dependent on the material of the regulating member, toner material, toner particle diameter and the environment of use.
Therefore, in order to attain a desired average value of the charge amount and a desired charge polarity, it has been necessary to arrange delicately the materials to be added to the toner. Methods for improving such a problem include a developing apparatus disclosed in Japanese Patent Laying-Open No. 10-63096 (hereinafter referred to as a first prior art example) and a developing apparatus disclosed in Japanese Patent Laying-Open No. 10-148999, in which among electric charges generated by discharge, those having a desired polarity are extracted from an electric charge generating apparatus and applied to the toner (hereinafter, this method will be referred to as electric charge supplying method).
The developing apparatus disclosed in the first prior art example will be described with reference to FIG. 11. FIG. 11 is a schematic diagram representing a structure of the developing apparatus disclosed in the first prior art example. A developing apparatus 80 is provided at a position opposing to a photoreceptor drum 91 as an image carrier. In a housing 90 of developing apparatus 80, there are a developer carrier 92 opposing to and close to photoreceptor drum 91 and conveying toner adhered on its surface, a layer forming member 93 regulating the toner on developer carrier 92 to form a toner layer, a stirring supply member 94 stirring the toner and supplying the toner to developer carrier 92, an electric charge supplying member 95 arrange opposing to developer carrier 92 and generating discharge at the opposing position, and a charge control member 96 arranged between electric charge supplying member 95 and developer carrier 92 to limit an electrolytic dissociation area of discharge generated therebetween.
Developer carrier 92 mentioned above is rotatably supported, and to which a DC voltage of about xe2x88x92200V and having the same polarity as the toner is applied. Thus, an electric field is formed between developer carrier 92 and photoreceptor drum 91, and the toner is transferred onto the latent image on photoreceptor drum 91. By the above-described method, the average value of the charge amount can be controlled in a relatively simple manner, and the distribution of the charge amount can be made sharper to some extent. Further, in the developing apparatus 80, discharge is generated in a small space formed by developer carrier 92 having a surface rubber layer with volume resistivity of 106 xcexa9xc2x7cm and layer forming member 93 formed of silicone rubber having volume resistivity of about 104 to 1010 xcexa9xc2x7cm. At this time, the charge amount in the toner layer on developer carrier 92 is reduced by the electric charges of both polarities generated, and thereafter, the toner layer is charged to a desired charge amount, by electric charge supplying member 95. Consequently, variation in the charge amount between the toner once charged by the electric charge supplying member 95, rotated once while not used for development of a latent image on photoreceptor drum 91 and charged again, and the toner used for development of the latent image on photoreceptor drum 91, supplied newly on the developer carrier 92 and charged once, can be reduced, and hence uniform charging not dependent on history becomes possible.
As the condition of discharging to reduce the charge amount in the toner layer, a frequency within such a range in that the toner cannot reciprocate following the oscillating electric field in the space, for example, a frequency of 3 kHz is recommended, which prevents adhesion of toner to the layer forming member 93.
As to the voltage, DC offset of 0V, a voltage at least twice the discharge starting voltage in the small space (for example, 1200V), and not higher than the voltage causing leakage because of high voltage (for example, at most 3000V) is recommended, so that electric charges of both polarities exist uniformly in the small space.
Further, a proposal of a structure of electric charge supplying member 95 for suppressing generation of reverse-charged toner is also described.
In the conventional method of friction charging, the charge amount of the toner is in proportion to the power of 1.5 to 2.5 of the diameter of particles, if the toner composition is comparable. Therefore, when the toner has small particle diameter, specific charge amount (charge amount/mass) becomes too large. For example, in the developing apparatus to which the friction charging method is applied shown in FIG. 10, when toner having average particle diameter of 9.5 xcexcm is introduced, the specific charge amount measured at the developing position is 35 to 40 xcexcc/g. When the toner having the same composition but average particle diameter of 5.5 xcexcm is used, the specific charge amount measured at the developing position is 65 to 68 xcexcc/g, and the toner has high specific charge amount. The toner having high specific charge amount causes a problem that density of a solid black image cannot be made sufficiently high. It is possible to attain a desired density by ensuring a potential difference for development in accordance with the specific charge amount. For this purpose, however, there would be considerable burden on the related processes and components, including setting of high charge potential of the photoreceptor drum.
Even in the electric charge supplying method, when the toner, especially the toner having small particle diameter is charged to an amount higher than the desired charge amount because of friction with the layer forming member 93 or the like in forming a toner layer on the developer carrier 92, it is impossible to adjust to the desired charge amount by the electric charge supplying apparatus. It may be possible to finely adjust the compositions of the toner and the layer forming member 93 such that the charge amount caused by the friction with the layer forming member 93 and the like to be the same or smaller than the desired amount and to supplement the shortage by the electric charge supplying apparatus. When the charge amount resulting from friction is to be reduced, however, it follows that the reverse-charged toner increases. Such a severe adjustment of the materials is against the desired object which is to be attained by the electric charge supplying method.
In this regard, by using the developing apparatus described in the first prior art example, by once reducing the charge amount of the toner layer and then newly charging again as described above, it is possible to generate a toner layer having a relatively small specific charge amount even when the toner particle diameter is small.
By such charge processes, it is possible to control the average value of the charge amount to a desired value in a relatively simple manner. Further, the distribution of charge amount can be made sharper to some extent, variation in the charge amount cycle by cycle can be reduced, and satisfactory image can be obtained.
The developing apparatus in accordance with the first prior art example, however, is still not free of the reverse-charged toner from the following reason. There is a friction between the toner and the layer forming member 93 or stirring supplying member 94, and in addition there is a friction between toner particles. Therefore, by the time the layer is formed, some parts of the toner surface are charged negative while others are charged positive. When the amount of electric charges of one toner particle is considered, some toner particles are charged to have the opposite polarity. When the toner layer that has 1) a normal charge polarity when viewed as a whole but containing toner particles partially charged to the polarity opposite to the normal charging polarity, and 2) reverse-charged toner particles, is charged by electric charge supplying member 95, the electric charges generated by corona discharge move along an electric line of force 120 as shown in FIG. 12 and adhere to the toner. At this time, when the reverse-charged portion of the toner is on a surface where adhesion of electric charges is possible (upper portion of the deposited toner particles), the electric charges of opposite polarity (in FIG. 12, positive polarity) are electrically eliminated, and charged to the normal polarity (in FIG. 12, negative polarity).
However, when the reverse-charged portion is on the surface where adhesion of electric charges is impossible (below deposited toner particles), the reverse polarity charges cannot be eliminated, even when electric charges of a single polarity are applied by corona discharge.
More specifically, it is possible for the toner facing the electric charge supplying member 95 to attain the desired charge amount, by applying the electric charges generated by the electric charge supplying apparatus. It is impossible, however, for the toner particles not on the surface, that is, the toner particles existing in the toner layer, to effectively receive the electric charges.
The electric charges generated by the corona discharge cannot reach that side of the toner particles even of the surface toner which are on the opposite side viewed from the electric charge supplying member 95. Therefore, the electric charges of reverse polarity of such a portion cannot be canceled. Thus, it is the case that the desired electric charges can be applied only to the surface portion.
The same applies to discharging by charge clearing at the small gap between the layer forming member 93 and the image carrier 92.
The phenomenon is studied intensively with respect to this problem, and the method and conditions for charging have been found that enable stable charging of toner having small particles and low specific charges, by eliminating reverse-charged toners and local reverse charging.
An object of the present invention is to provide a developing apparatus capable of making smaller a distribution range of charge amount, particularly capable of reducing reverse-charged toners without requiring delicate arrangement of component materials of the developer, and improving stability of development and image quality by forming a toner layer of small particle size and low specific charges.
The above described objects can be attained by a developing apparatus in accordance with one aspect of the present invention, that is, a developing apparatus supplying a developer to an image carrier, including a developer carrier carrying a developer on its surface, a developer regulating member regulating layer thickness of the developer, and a charge amount control member controlling the charge amount of the developer, wherein the charge amount control member is provided downstream in the direction of movement of the developer than the layer thickness regulating member regulating the layer thickness of the developer and electrically insulated from the developer carrier, and by causing flight of the developer over the developer carrier, by an AC voltage applied between the developer carrier and the charge amount control member, electric charges generated by electrolytic dissociation of the gas caused by the AC voltage are applied to control the amount of electric charges of the developer.
In this structure, the developer carrier carrying the developer on its surface and the developer regulating member regulating the layer thickness of the developer carried by the developer carrier are electrically insulated, and by the AC voltage applied therebetween, the developer is caused to fly in the small space formed near the contact portion from the developer carrier, so that the electric charges generated by gas electrolytic dissociation caused by the AC voltage are applied and the charge amount of the developer is thus controlled.
Therefore, the developer flies in the small space where electric charges have been generated by gas electrolytic dissociation caused by the AC voltage, whereby the developer reciprocates in the small space where the electric charges generated by the gas electrolytic dissociation exist. As a result, such a control becomes possible in the that the whole developer comes to have uniform charge amount by the positive and negative electric charges abound around the developer. Further, the distribution range of the charge amount is made smaller, reverse charging of the developer is eliminated, and charging with an appropriate specific charge amount is done without necessitating delicate arrangement of the component materials of the developer, whereby development is performed stably and the image quality can be improved.
The present invention includes an electric charge supplying apparatus provided downstream along the direction of movement of the developer carrier than the developer regulating member, for applying electric charges to the developer on the developer carrier, and the developer regulating member can also serve as a charge amount control member. By this structure, it becomes possible to effectively control the charge amount of the developer, in the small space near the developer regulating member.
According to an embodiment of the developing apparatus of the present invention, the developer regulating member is formed of a rotating body and the apparatus further includes a developer removing member that is in contact with the developer regulating member and removing the developer carried by the developer regulating member, and a charge clearing means arranged upstream along the direction of rotation of the developer regulating member than the contact portion between the developer regulating member and the developer carrier and downstream along the direction of rotation of the developer regulating member than the contact portion between the developer regulating member and the developer removing member, for clearing the charges on the surface of the developer regulating member.
By this structure, in the present invention, the surface of the developer regulating member is cleaned by the developer removing member and further, the surface potential of the developer regulating member is cleared by the charge clearing means, before the step of developer charge clearing in the small space. Therefore, at the time of charge clearing with the developer flying in the small space, the charge clearing operation can be done stably.
Preferably, the developing apparatus of the present invention includes an electric charge supplying apparatus provided downstream along the direction of rotation of the developer carrier than the contact portion between the developer carrier and the developer regulating member and upstream along the direction of rotation of the developer carrier than the position at which the developer carrier and the image carrier oppose to each other, for supplying electric charges to the developer layer on the developer carrier, and the charge clearing means clears charges, using the electric charges generated by the electric charge supplying apparatus. By this structure, it becomes possible to charge the developer on the developer carrier to a prescribed potential by means of the electric charge supplying apparatus, as the electric charges generated within the electric charge supplying apparatus are used, a stable charge clearing level can be attained without the necessity of providing a new charge clearing apparatus or a power supply.
In the present invention, more preferably, the developer carrier and the developer regulating member move in opposite directions at the contact portion, and the peripheral speed of the developer regulating member is set to be faster than the peripheral speed of the developer carrier. When the peripheral speed of the developer regulating member is slower than the peripheral speed of the developer carrier, new surfaces of the developer carrier move successively to the discharging portion, while the surface that has been already exposed to the discharging portion of the developer regulating member opposes thereto, and therefore, the charge clearing property of the developing layer becomes unstable, because of the influence of the charge state of the developer regulating member. When the peripheral speed of the developer regulating member is made faster than that of the developer carrier, a new surface that has not yet been exposed to the discharge of the developer regulating member opposes to the developer layer, and hence, charge clearing property is made stable.
In the present invention, when the developer is caused to fly from the developer carrier to the small space by the AC voltage applied between the developer carrier and the developer regulating member and the developer has its charges cleared by the application of the electric charges generated from gas electrolytic dissociation caused by the AC voltage, it is preferred that the absolute value of the specific charge amount is at least 5 xcexcC/g. This enables formation of the developer not including reverse-charged developer on the developer carrier.
In a preferred embodiment of the present invention, the developer carrier has a multi-layered structure having an elastic layer and a conductive layer formed in this order around a conductive axis of rotation, and a conductive layer electrically connecting the conductive layer and the axis of rotation is formed at an end surface.
By this structure, it becomes possible to set the surface potential of the developer carrier to approximately 0V by grounding the axis of rotation, for example, and hence, it becomes possible to stably charge the developer.
According to another aspect, in the developing apparatus of the present invention, the charge amount control member is provided downstream along the direction of movement of the developer than the layer thickness regulating member regulating the layer thickness of the developer and electrically insulated from the developer carrier. An AC voltage is applied between the developer carrier and the charge amount control member, wherein Vp/(square of f) greater than 160 is satisfied where Vp (V) represents pulsating amplitude of the AC voltage and f represents frequency f (kHz), and wherein the AC voltage is not lower than a discharge start voltage in the space formed between the developer carrier and the charge amount control member.
In this structure, as the developer is caused to fly in the small space where electric charges generated by the gas electrolytic dissociation caused by the AC voltage exist, and the developer reciprocates in the small space where the electric charges generated by the gas electrostatic dissociation exist. As a result, it becomes possible to perform such a control in that the developer as a whole comes to have uniform charge amount, because of the positive and negative electric charges abound around the developer. Further, the distribution range of the charge amount is made smaller, reverse charging of the developer is eliminated and charging with an appropriate specific charge amount is performed without the necessity of delicate arrangement of the component materials of the developer, whereby development can be done stably and the image quality can be improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.